Powdery mildew outbreaks caused by Podosphaera macularis on Hop cultivars possessing the resistance gene R6 in the Pacific Northwestern United States

Authors: WOLFENBARGER, S - Oregon State University; ECK, E - Oregon State University; OCAMB, C - Oregon State University; PROBST, C - Washington State University; NELSON, M - Washington State University; GROVE, G - Washington State University; Gent, David - Dave

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/4/2013
Publication Date: 6/30/2014
Citation: Wolfenbarger, S.N., Eck, E.B., Ocamb, C.M., Probst, C., Nelson, M.E., Grove, G.G., Gent, D.H. 2014. Powdery mildew outbreaks caused by Podosphaera macularis on Hop cultivars possessing the resistance gene R6 in the Pacific Northwestern United States. Plant Disease. 98(6):852-853.

Interpretive Summary: Resistant cultivars offer an efficient means to manage plant disease. In the Pacific Northwestern U.S., hop cultivars have been developed that were immune to the disease powdery mildew. One such gene, R6, that confers such immunity was utilized extensively in breeding programs and provided high levels of resistance in the field for nearly 15 years. This brief communication details the emergence and identification of virulent strains of the pathogen able to overcome the resistance conferred by the gene R6. The increasing prevalence of virulent strains of the pathogen and outbreaks of powdery mildew on formerly resistant cultivars necessitates changes in breeding strategies and disease management efforts to minimize damage resulting from the disease.

Technical Abstract: Resistant cultivars of hop (Humulus lupulus) have been grown, with the aim of helping to manage powdery mildew in the Pacific Northwest since the first report of the disease in the field in 1997 (4). A major objective of many breeding programs is development of resistance to powdery mildew, and this has generally been achieved by single resistance genes (qualitative resistance). One such gene, R6 (3), has been utilized extensively in new cultivars and prevented epidemics of the disease in those cultivars across the Pacific Northwestern U.S. for approximately 15 years. In 2011, a grower in Washington State reported outbreaks of powdery mildew on cv. Apollo, which is thought to possess powdery mildew resistance derived from R6. Fungicides and cultural control measures were applied, and the grower reported no substantial crop damage from the disease. During the winter of 2012, the grower planted rhizomes of cv. Apollo in a greenhouse in the Yakima Valley of Washington State and later found the plants to be affected by powdery mildew. Affected leaves from plants of cvs. Apollo, Newport, and Nugget (all thought to possess R6-based resistance) grown in the same greenhouse were later provided to the authors. Conidia from affected plants were transferred to plants of the highly susceptible cv. Symphony, which is not known to contain any resistance genes. After 10 to 14 days of incubation, resultant conidia from each cultivar above (total of 3 isolates) were transferred to greenhouse grown plants of cvs. Nugget and Symphony and incubated at 18°C. Within 7 days, powdery mildew colonies with characteristic non-branching conidiophores and barrel-shaped conidia typical of P. macularis (2) were apparent on both cultivars from all isolates. Cleistothecia did not develop in any of the inoculations. In addition, Nugget and Symphony plants were inoculated with a field population of P. macularis originating from cultivars lacking R6 in Oregon. These inoculations on Nugget did not develop powdery mildew whereas Symphony plants did. Non-inoculated controls remained free of powdery mildew. Results were identical in two additional experiments. The sequence of the mating type idiomorph, MAT1-1, was obtained to confirm identity of the pathogen as P. macularis as described previously (1). The sequences were identical among the three isolates obtained from the greenhouse in Washington and isolates of P. macularis obtained previously from Oregon and Washington. MAT1-2 idiomorph was not detected in the isolates collected. While R6-virulent strains have been detected previously in race characterization experiments, these strains have not caused widespread epidemics of powdery mildew. The increasing prevalence of virulent strains of P. macularis and outbreaks of powdery mildew on formerly resistant cultivars necessitates changes in breeding strategies and disease management efforts to minimize damage resulting from the disease. The distribution of virulent strains of the pathogen and susceptibility of formerly resistance cultivars to powdery mildew are currently under investigation.